In recent years, an HEV (Hybrid Electric Vehicle) developed for the purpose of protection of environment and improvement of performance has come into wide use. The HEV is equipped with two drive sources such as an engine and a motor, and an inverter which converts a direct current of a secondary battery into an alternating current to supply the alternating current to the motor. In the HEV, it is necessary to cool the motor and the inverter, in addition to an existing engine cooling system. Accordingly, the conventional HEV has a cooling circuit for cooling an engine (an engine cooling circuit) and a cooling circuit for an HEV system (an HEV cooling circuit), and thus cools the engine or the HEV system by circulating coolant in the associated cooling circuit. However, since a coolant control temperature (for example, 85° C. to 95° C.) of the engine cooling circuit differs from a coolant control temperature (for example, 35° C. to 50° C.) of the HEV cooling circuit, the engine cooling circuit is independent from the HEV cooling circuit.
In the HEV such as a hybrid bus using an automated manual transmission (AMT), a shift synchronizer is controlled by controlling torque of a motor. For this reason, since the shift synchronizer is not controlled when output of the motor is not performed, the HEV may not travel.
Thus, when the HEV system is overheated due to a lack of coolant in the HEV cooling circuit in the related art, output of the drive motor and the inverter is restricted or inhibited, thereby causing the synchronizer of the AMT to be not controlled so that the HEV may not travel.